As illustrated in FIG. 1, the major components of a thermal dye transfer printing system are:
1. The print head 10, which contains an array of discrete resistors to supply heat or electrodes to provide current with the heat generation via Joule heating. PA1 2. The donor sheet 12 which consists of a thin base film carrying a dye material on one side and a slip layer on the side sliding against the print head. For Joule heating in the belt, a current return layer is required. The base has to be electrically conductive. Sheet 12 is fed between donor supply 11 and donor take-up 13. PA1 3. A receiver material 14 (such as paper or transparency) in intimate contact with the dye side of the donor sheet. PA1 4. A platen roller 16 required to form an intimate contact nip between the print head, the dye donor and image receiver, to enable transfer of the dye from the donor to the receiver, when the pulsed heat is generated either in the ribbon 12 or the print head 10. PA1 1. The dye donor element in a thermal printing system can be reused, reducing cost and complexity of the system.
FIG. 2 shows resistive ribbon printing where electrodes 18 inject current into the donor ribbon 20 where it heats the ink 22 and transfers it to the receiver 24.
A significant problem in this technology is that the dye donor members used to make the thermal prints are generally intended for single (one time) use. Thus, although the member has at least three times the area of the final print and contains enough dye to make a solid black image, only a small fraction of this dye is ever used.
After printing an image, the dye donor member cannot be easily reused, although this has been the subject of several patents. The primary reason that inhibits reuse of the dye donor members is that the dye transfer process is very sensitive to the concentration of dye in the donor layer. During the first printing operation, dye is selectively removed from the layer thus altering its concentration. In subsequent printings, regions of the donor member which had been previously imaged have a lower transfer efficiency than regions which were not imaged. This results in a ghost image appearing in subsequent prints.
The cost associated with having a single use donor ribbon is large because of the large area of ribbon required, as well as the large excess of dye remaining coated on the donor member. While this technology is able to produce high quality continuous tone color prints, it is desired to provide an approach which has all of the good attributes of thermal dye transfer imaging but without the limitations associated with single use donor members.
Some work has been done by others to accomplish similar goals. For example, U.S. Pat. No. 5,286,521 discusses a reusable wax transfer ink donor ribbon. This process is intended to provide a dye donor ribbon that may be used to print more than one page before the ribbon is completely consumed. U.S. Pat. No. 4,661,393 describes a reusable ink ribbon, again for wax transfer printing. The ink ribbon contains fine inorganic particles and low melting waxy materials to assist in the repeated use of this ribbon. U.S. Pat. No. 5,137,382 discloses a printer device capable of re-inking a thermal transfer ribbon. However, again the technology is wax transfer rather than dye transfer. In the device, solid wax is melted and transferred using a roller onto the reusable transfer ribbon.
U.S. Pat. No. 5,334,574 describes a reusable dye donor ribbon for thermal dye transfer printing. This reusable ribbon has multiple layers containing dye which limit the diffusion of dye out of the donor sheet. This enables the ribbon to be used to make multiple prints. In addition, the ribbon may be run at a slower speed than the dye receiver sheet, enabling additional utilization. U.S. Pat. No. 5,118,657 describes a multiple use thermal dye transfer ink ribbon. This ribbon has a high concentration dye layer on the bottom and a low concentration dye layer on the top. The low concentration dye layer meters or controls dye transfer out of the ribbon. This enables the ribbon to be used multiple times. U.S. Pat. No. 5,043,318 is another example of a thermal dye transfer ribbon which can be used multiple times.
U.S. Pat. No. 5,090,828 discloses a dielectric transfer technology for replenishing a donor sheet with small toner-like particles, filling in the regions where mass has been transferred from the ribbon.
U.S. Pat. No. 4,713,281 describes a multiple use pressure sensitive transfer recording medium.
U.S. Pat. No. 4,894,283 relates to a reusable thermal mass transfer ribbon consisting of carbon black, hydrocarbon wax and a thermal plastic resin.
U.S. Pat. No. 4,865,913 shows to a multiple use thermal mass transfer ink sheet, having an ink holding, porous membrane layer filled with hot melt ink.
U.S. Pat. No. 5,376,619 teaches a thermal dye transfer process in which a donor ribbon may be run at a slower speed than a receiver, thus generating a greater utilization of the donor ribbon.
U.S. Pat. No. 5,347,344 describes an electro-photographic type process, in which waxy toner particles are transferred in an image-wise fashion onto a reusable donor ribbon. The donor ribbon then subsequently transfers the toner image onto a receiver sheet using a thermal print head.
U.S. Pat. No. 4,504,840 discloses a reusable ribbon which is re-inked after printing by transferring wax transfer type colorant back to the ribbon in the liquid state. This method also discloses the use of a resistive ribbon type printing technology.
U.S. Pat. No. 4,414,555 relates to a process in which a mass transfer type printing ribbon is used to transfer an image to a receiving sheet. The used ribbon is then re-inked in this process, enabling it to be reused.
Accordingly, none of these disclosures directly relate to the concept of the re-application of dye to a dye donor ribbon. Techniques have been used in wax transfer printing to replenish a used donor ribbon. However, in such techniques, a mass transfer of wax back onto the donor sheet is used, rather than a re-diffusion of dye back into the donor layer. Similarly, the concept of replenishing a resistive ribbon thermal transfer sheet has been disclosed, but again only in the context of wax transfer imaging. There are several disclosures for reusable thermal dye transfer ribbons. However, these ribbons attempt to control the diffusion of dye out of the ribbon so that they could print multiple times, rather than replenishment of the dye in a dye donor element.